The invention relates to an electronic circuit, and in particular to a variable gain amplifier circuit.
It is desirable in some situations to provide a variable gain amplifier, in which the gain varies logarithmically with a control signal, to produce an output signal which produces a linear change in output signal level measured in dB, in response to a linear change in input signal level.
U.S. Pat. No. 5,572,166 describes a variable gain amplifier, in which a linear change in a gain control current produces an exponential change in gain, thereby providing linear-in-decibel gain control.
U.S. Pat. No. 4,816,772 discloses a variable gain amplifier with multiple cascode amplifier stages, connected in cascade. A control voltage is applied to a linearization circuit, the output of which is input to a voltage controlled voltage source, which produces a conditioned control voltage. The conditioned control voltage is fed back to the linearization circuit and is provided to each of the multiple cascode amplifier stages. The conditioned control voltage controls the amplifier stages such that a linear change in the control voltage produces a linear change in dB of the overall gain of the circuit.
In accordance with one aspect of the invention, a control signal is conditioned such that it produces a conditioned control signal which is the sum of an exponentially varying component and a constant component. The resulting conditioned control signal, applied to an amplifier circuit, produces a gain which varies linearly in dB with changes in the control voltage.
In accordance with a second aspect of the invention, a control signal is applied to an amplifier made up of a plurality of variable gain attenuator stages, the control signal having been conditioned such that it produces a gain which varies linearly in dB with changes in the control voltage.